The present invention relates generally to an electrophotographic printing machine, and more particularly, concerns a system for transforming scanner RGB data into a colorimetric system. The invention is applicable to the transformation of RGB data into colorimetric systems such as CIELAB or CIELUV.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image is developed by bringing toner into contact therewith. This forms a toner image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a path internal the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray 4 subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal the printing machine where a plurality of toner images is transferred thereto and then to an output tray for subsequent removal. With regard to multicolor printing, a sheet gripper secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The sheet gripper grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
Current 3-color scanners function by interposing a set of color filters in front of a broad spectrum photosensitive device. This filter set is usually chosen to be set sensitive to short medium and long wavelength regions of the spectrum (or B, G, R for short). The information produced by the scanner is usually further processed and eventually sent to a printer. In processing the scanned image for presentation to the printer, it is desirable to have the color information in a colorimetric system, such as CIELAB or CIELUV, rather than in terms of the scanner primary colors.
U.S. Pat. No. 4,745,467 to Sekizawa et al. discloses a color image processing apparatus which comprises a color line image sensor, a matrix circuit and a color conversion circuit. The sensor reads a document to output a plurality of fundamental color signals. The matrix converts the signals into a brightness signal and two color difference signals. The color conversion converts the brightness and color difference signals into ink amount control signals. Non-linear conversion circuits are provided to change low level input signals greater than high level inputs.
U.S. Pat. No. 4,409,614 to Eichler et al. discloses a method for reproducing originals in which originals are first scanned and converted into three primary signals having actual color values in a predetermined color coordinate system which are corrected according to visual sensitivity. Primary signals derived from this are then converted to another color coordinate system, have gradation changes or any other non-linear process.
U.S. Pat. No. 4,402,015 to Yamada discloses a picture signal pre-processing method for a picture reproducing machine in which an original picture is scanned photoelectrically to obtain a picture signal. First conversion characteristic data stored in a memory is read out and then changed by second conversion characteristic data depending on a desired reproducible density range of the original.
U.S. Pat. No. 4,727,425 to Mayne et al. discloses an image reproduction system in which digital signals are generated for each pixel of an image to be reproduced corresponding to color values. The values are applied to a set of look up tables to determine if the values fall within a certain range. The look up tables provide an output representing one of a plurality of processing operations for that pixel. The operations may be retouching, replacement tinting, or dropout. The tables define color ranges within which processing operations are required.
U.S. Pat. No. 4,500,919 to Schreiber discloses a color reproduction system for reproducing an original comprising a scanner for producing a train of tristimulus appearance signals, aesthetic correction circuitry for introducing alterations into the signals to provide modified appearance signals, and a colorant selection mechanism for selecting corresponding reproduction values representing values of colorants to produce a color match on a medium of the appearance signals. A model is used which corrects for unwanted absorptions of inks followed by processing in a matrix. After processing, a look up table is used to remove any error.
While the related art recognizes the need for processing the scanned color images, the art does not provide an efficient an effective system for converting RGB scanner data into a colorimetric system via transformation of the raw RGB signals into END-like RGB signals.